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ON NOVEMBER, 18, 1755, THE EARTH beneath the waters off Cape Ann heaved. Within seconds, the seismic waves generated there traveled to the twisting lanes and wharves of pre-Revolutionary Boston. According to historical accounts, chimneys toppled from roofs, steeples parted from churches, and gables crumbled from building fronts and shattered on the lanes below. The weather vane atop Faneuil Hall snapped. Vibrations were felt from Halifax, Nova Scotia, to Chesapeake Bay. Estimated at a magnitude 6.2, the Cape Ann earthquake is one of New England's strongest in recorded history.

But it's not the aberration you might think. In an average year, 30 to 40 earthquakes strike New England. Only a small fraction are strong enough to detect without instruments, yet a few have been large enough to cause damage. Big quakes outside New England can be felt here, too. Four years ago, a magnitude 5.2 tremor that jolted a remote region in the Adirondack Mountains, causing roads to collapse and foundations to crack, was felt as far south as Maryland. In 1929, a magnitude 7.2 quake centered off the southern coast of Newfoundland generated a tsunami that killed at least two dozen people; the quake was felt as far away as New York City.

A study by Boston College seismologist John Ebel, published earlier this year, zeroes in on the epicenter of the 1755 Cape Ann earthquake. Ebel, who has spent the last 25 years studying local quakes, also proposes a new idea: that all New England earthquakes -- including the 1755 one and a 1638 magnitude 7 quake probably centered in New Hampshire -- might be the aftershocks of an even larger historical quake. He says another large one may be looming.

Our earthquake threat is made more pressing by what distinguishes Boston among American cities: its elegant brick-and-mortar architecture, which in many cases sits on loose, unstable soil. Experts also warn that the city's aging infrastructure and utilities -- sewer mains, gas lines, bridges, and overpasses -- are rife with vulnerabilities. The length of time since the last significant quake seems to have dulled -- and, in some cases, erased -- our perception of the threat. Yet earthquakes do happen here. Why, then, isn't Boston ready for the next one?

TO UNDERSTAND BOSTON'S COMPLEX SET OF risks, we must first understand the ground beneath our feet. Most quakes, says Ebel, who directs BC's Weston Observatory, occur at the boundaries of tectonic plates, great continent-carrying masses of land. But Boston sits more than 1,000 miles from the edge of the North American plate. Using seismic data collected over the last 75 years at the observatory and historical records, including church logs and newspapers, Ebel and his colleagues estimate that moderate (magnitude 5) quakes happen every 50 to 90 years in New England. He extrapolates that magnitude 6 quakes, like the one in 1755, occur in the region every 500 to 900 years.

Scientists once believed that New England earthquakes were caused by the slow, and occasionally jarring, upward movement of the region after the retreat of the glaciers 10,000 years ago. But current data suggest that our quakes result from the collision of the North American and Pacific plates, which puts unrelenting pressure on the continent. And, of course, Ebel has added a new wrinkle - that New England's quakes might be aftershocks of a larger historical event. "It's a very new idea," he says. "But the largest quakes that are possible in New England may still be on the horizon."

Whatever the size of the quake, says Nano Seeber, a seismologist at Columbia University's Lamont Doherty Earth Observatory, its effects are magnified by the way that earthquakes travel through the rocks of the Northeast. Rock formations here tend to be more dense, rigid, and cold and less fractured than rocks in, say, California. So seismic waves generally travel farther. It also appears that a greater proportion of Northeast quakes happen near the surface, so smaller tremors have the potential to do greater damage. "In California, damage happens at 5.5," Seeber says. "Here, it can happen at 4.5."

"Amplification" and "liquefaction" are two words often used by specialists to describe Boston's earthquake vulnerability. Amplification -- when seismic waves hit a region of loose soil and become stronger -- was demonstrated to horrific effect in the Mexico City quake of 1985; a strong earthquake centered hundreds of miles away was strengthened at the surface by the soft soils beneath the city. Thousands of unreinforced buildings were leveled, and 9,500 people were killed.

Liquefaction happens when saturated sand or gravel deposits are shaken, increasing the water pressure between the grains and causing the soil to flow freely. When soil liquefies, buildings may sink as though into quicksand and can buckle as their foundations shimmy and shift; that's what happened in the Loma Prieta quake of 1989, which killed at least 63 people in the San Francisco Bay area. A 2004 study of Boston by researchers Charles Brankman of Harvard University and Laurie Baise of Tufts University shows the greatest threat of liquefaction to areas built on fill, such as Logan Airport, Back Bay, South Boston, and the Cambridge waterfront.

THE FILLING OF MARSHES and shallow waters helped Boston prosper. "It's hard to conceive of Boston and, in fact, almost any big city in the United States, without the process of land making," says Nancy Seasholes, a research fellow at Boston University and author of Gaining Ground: A History of Landmaking in Boston. However, land making is a fundamental reason why many of Boston's buildings are not as stable as they look. The stately row houses that line Tremont Street in the South End, Charles Street at the foot of Beacon Hill, and Commonwealth Avenue in Back Bay are anchored by wooden pilings driven into the soil like candles into a moist birthday cake.

And the cake is getting stale.

Paul Finnegan awoke in his Beacon Hill bedroom on December 18, 1986, to a crash of brick and mortar next door. "It was the middle of the night," he recalls. "My head was right against the adjoining wall, and I heard this massive crash. I bolted upright to see if it was an earthquake." It was no earthquake but his neighbors' home buckling above its rotted foundation piles. "They were locked inside. They couldn't get their doors and windows open. It was a real mess." The level of water in the soil had fallen, and the wooden pilings, exposed to air, had rotted and failed, causing the building to settle nearly half a foot. (Most pilings in that area reach about 30 feet into the ground.)

With Boston's complex history of construction, large volumes of water have either leaked or been pumped out of the ground. Yet this ground water is crucial in preventing dry rot and insect infestation of the wooden pilings that hold up many of the city's older buildings. The falling water table has left the foundations of many of Boston's historic buildings compromised and susceptible to earthquakes and other disturbances.

These landfill problems -- amplification, liquefaction, and dry rot -- are compounded by the fact that Boston's grand old stock of wood-frame row houses, red-brick Victorians, and other unreinforced masonry buildings are, by almost every measure, structurally defenseless against the ground movement experienced in a large earthquake. Old buildings in Boston's suburbs face less risk, since fewer are built on landfill.

WE CAN GUESS THE TOLL AN EARTHQUAKE would exact on Boston. In 1990, researchers predicted that a magnitude 6.2 quake would cause $2 billion to $10 billion in damage in the Boston area, along with "hundreds of deaths and thousands of major and minor injuries." An unpublished 1997 study of Boston forecast similar results. (The Loma Prieta quake caused about $10 billion in damage.) And in 1997, a Stanford University researcher concluded that -- given the high population density, large proportion of fragile buildings, and precarious soil conditions -- the risk of a quake causing catastrophic damage was greater in Boston than in San Francisco or Mexico City.

The city's most fragile buildings went up before seismic building codes were introduced to the city in 1973 and adopted statewide in 1975. Rolf Goetze, a researcher at the Boston Redevelopment Authority, estimates that about three-quarters of the city's 100,000 buildings were constructed before 1973. He adds that the city doesn't track how many of these older structures have been brought up to at least 1973 building standards.

While many of Boston's dwellings and low-rise brick structures lack strong lateral systems that compensate for forces like a quake that move a building from side to side, the modern skyscrapers that punctuate the city skyline are equipped with these key systems. "Lateral force requirements came into effect in Massachusetts with the seismic code changes in the 1970s," says Dominic Kelly, a structural engineer at the Waltham offices of Simpson, Gumpertz and Heger who specializes in earthquake retrofitting and seismic design. For one example of a building representing the leading edge of seismic engineering, Kelly points to Cambridge. There you'll find Simmons Hall, an MIT dormitory that looks like a giant Lego building. Throughout the concrete structure are shear walls -- reinforced barriers placed to prevent a building from being pulled apart, or sheared, internally by side-to-side movement.

If only Boston's underground displayed such revolutionary design. Many of the aging gas, sewer, and water mains beneath the city were built long before there were seismic provisions in the building code. "There you have a real problem," says John Christian, an engineering consultant in Boston. "To lose water for a few days is one kind of dilemma, but if you need that water to put out fires, like they did in San Francisco in 1906, you could have a real problem." And the city's roads, tunnels, and bridges? "The good news is that the majority of these structures have been built up to seismic codes," Christian says. "The bad news is that these things have, by all accounts, not been properly maintained and therefore are more vulnerable to disturbances." One benefit of the Big Dig, says Christian -- who worked as a consultant on the highway project after water was found leaking into the tunnels -- is that outdated utilities and infrastructure have been replaced with the most current, earthquake-resistant designs. "The Dig is certainly one project in the city where the latest seismic provisions have been incorporated."

EARTHQUAKES COME WITH LITTLE warning. In most cases, mere seconds. Seismologists are working to develop a system that would alert vulnerable areas days or months before. Nascent alarm systems can do such things as detect seismic waves and shut off gas lines, but good building design is the best defense against quakes, authorities say.

Yet Boston has been slow to turn the evidence and predictions of engineers and seismologists into action. While Massachusetts's building codes are adapted from California's -- the gold standard -- the effort to protect the city's historic landmarks and provide incentives to property owners to retrofit vulnerable buildings is embryonic, at best.

California planners and seismologists are not surprised that New Englanders have ignored earthquake preparedness. Susan Tubbesing, executive director of the Earthquake Engineering Research Institute in Oakland, says that even in earthquake-prone California, retrofitting is a hard sell. "We are most concerned with the large number of unreinforced masonry buildings, offices, and apartments that remain in California," she says. "These are the kind of buildings that 'pancaked' in the 1989 Loma Prieta and 1994 Northridge earthquakes."

While San Franciscans have spent billions in retrofitting over the last 20 years, 5.8 percent of San Francisco County's single-family housing stock and 8.7 percent of its multi-family stock are built of unreinforced masonry, according to the Association of Bay Area Governments. "San Francisco has lots of these buildings, but there is no way to force businesses and homeowners to come into compliance," Tubbesing says. Ken Topping, former director of city planning for Los Angeles, says that retrofitting an old building can cost more than $100 per square foot. "Owners of larger buildings and retail space often find it difficult to make the changes," he says. In 1999, city planners abandoned a retrofitting mandate in San Bernardino because they were afraid it would hurt small-business owners and stunt economic growth.

The insurance picture also shows public apathy. Despite predictions that there is a 62 percent chance of San Francisco experiencing a serious tremor in the next 30 years, only 13 percent of Californians own earthquake insurance. "In New England, the percentage of earthquake insured is surely far lower," says Jeanne Salvatore, a spokeswoman for the Insurance Information Institute in New York City.

Certainly, retrofitting every seismically deficient building in Boston would be a logistically Herculean and financially crippling task. At current rates of retrofitting and new construction, says Ebel of the Weston Observatory, half of the city's buildings will meet seismic standards by 2036. Only time and geology will tell whether this is fast enough. However, a more focused push could be made to coordinate a citywide response and bring Boston's landmark buildings into compliance. If cost and time will not allow the city's vulnerable buildings to be retrofitted, then the people inside should be made aware by collecting structural information in a central database that is easily accessible to renters and home buyers. Owners can reduce their risk by having their homes inspected for major structural weaknesses like rotting piles and foundation cracks. The first step in preparing for an earthquake is to acknowledge the temblors' tremendous, transformative power -- as well as their place in history -- even in spots where the ground appears deceptively firm.

What Are the Odds?
With a major earthquake striking New England every 500 to 900 years, bookmakers would say the odds of one happening tomorrow are 1 in 255,500. That's better than the odds of being hit by lightning this year, 1 in 700,000, or of getting a royal flush in a game of five-card stud, 1 in 649,740. Not betting odds, you say? By comparison, the odds of hitting the Powerball jackpot are 1 in 146,000,000 -- about the same chance as an earthquake striking while you read this.

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